I just happened to be there yesterday....I would suggest the many different types of Tofu products, Fresh Packaged Noodle and Miso Paste which were on sale. For Vegetables, try the many cabbages or Asian offering not readily available in your normal markets. They also have a great selection of unique mushrooms as well.

For prepared foods/Sashimi....both Negitoro and Chaka Ika Sansai were also on sale . Two small packages amounted to less than $5....a great little snack.

I've only been to Mitsuwa once but have gone to Japanese markets elsewhere for several years.

I'd buy Japanese pickles, miso, mushrooms (though not after Fukushima), burdock, onigiri (salmon, tuna), ikura, uni, seafood of all sorts (for broiling, soups, or sushi/sashimi), kombu and katsuobushi for dashi, other seaweeds, mirin, and snacks. I buy rice in regular markets as it can be expensive in Japanese ones.